Edited version of the Flat-Plat Gnome Shell theme I made to match the GTK better and be even flatter. I give all credit to the original themes creator, and recommend you download that theme & the matching gtk first. (http://gnome-look.org/content/show.php/Flat-Plat?content=167704)

Im relatively inexperienced at this, but Id love to take a shot any suggestions you have! Last changelog:

First is by using the official PPA of the theme. Some big projects, like Numix, provide their own PPA for easily installing the themes. For example, you can use the following commands to install Numix theme in GNOME Shell

Not every theme designer provides a PPA. But you can always download theme pack which are often compressed. All you have to do is create a .themes directory in your home directory and extarct the theme files there.

You can do it graphically using the file manager Nautilus. Go to Home, right click, make new folder and name it .themes. Mind the dot at the start of the file name which is very important here